Thermistors

ABSTRACT

Thermistors are provided in which the thermally sensitive semiconducting material has the formula:

1.5. .1 F eld 9!. earch:

United States Patent [191 De Vries et a1.

[ THERMISTORS I 75 I Inventors: Robert C. De Vries, Burnt Hills;

James F. Fleischer, Scotia, both of [75 T Assignee: General ElectricCompany,

Schenectady, NY.

22 Filed: Dec. 26, 1972 [21 Appl. No.: 318,364

Related US. Application Data [62] Division of Ser. No. 155,949, June 23,1971.

[52] US. Cl. 338/22, 252/518 [51] Int. Cl H0lc 7/04 [56] ReferencesCited UNITED STATES PATENTS 2,700,720 1/1955 Torok 29/612 X RESISTANCEIN OHMS CURVE A CURVE 5 CURVE C CURVE D CURVE E CURVE F Oct. 16, 19733,231,522 1/1966 Blodgett et a]. 252/521 X Primary Examiner-C. L.Albritton Attorney-Gerhard K. Adam [5 7] ABSTRACT Thermistors areprovided in which the thermally sensitive semiconducting material hasthe formula:

wherein Z is a member selected from the group consisting of chromium,molybdenum, tungsten, and sulfur or mixtures thereof. The thermistordevice can be made by dipping two closely spaced electrical conductorsinto a melt of the thermally sensitive-semiconducting material to forman element of the material between the conductors.

8 Claims, 1 Drawing Figure RES/STANCE vs TEMPERATURE CURVES FOR SINGLECRYS 741.5 0F PDQ/M400 "PbOPbC/Q;

TEMPERATURE 1C) PAIENTEDum I6 1975 RESISTANCE IN OHMS RE 5/5 TA/VCE VsTEMPERATURE CURVES FOR SINGLE CRYSTALS 0F Pb0'PbMo0 Pb0-PbCr0 CURVE ACURVE B CURVE C CURVE 0 CURVE E CURVE F 4 l l I I I I I I I I I ITEMPERATURE (C) THERMISTORS This is a division, of application Ser. No.155,949, filed June 23, 1971.

THERMISTORS Thermistors are semiconducting devices having electricalconductivities between those of conductors and insulators. The termthermistors is derived from thermally sensitive resistors, since theresistance of a thermistor varies rapidly with temperature. In recentyears temperature measurements with semiconducting, thermally sensitiveresistors have become increasingly widespread. A discussion of suchdevices is given by EC. Robertson et al., Properties of Thermistors Usedin Geothermal Investigations and by R. Rasbet et al., Preparation ofThermistor Cables Used in Geothermal Investigations, Geological SurveyBulletin l203-B,C, U.S. Government Printing Office, Washington (1966).

The advantages of thermistors over resistance thermometers and thethermocouple is described by CR. Droms, Thermistors for TemperatureMeasurements, in Am. lnst. Physics, Temperature, Its Measurement andControl in Science and Industry, New York, Reinhold, v.3, part 1, pages339-346. The semiconducting materials used in the latter report were theoxides of nickel, manganese, and cobalt.

Lead pigments such as red lead, basic lead sulfate and basic leadchromate are among the traditional raw materials of the paint industry.These ingredients have been used particularly in anticorrosive primerformulations for the protection of steclwork. The lead pigments arediscussed in some detail under Pigments (Inorganic), Encyclopedia ofChemical Technology, 2nd Edition, Vol. 15, pages 495-555 (1968).

Quite surprisingly, we have discovered that certain classes of leadpigments have semiconducting properties. Furthermore, the resistance ofthese materials varies rapidly with changes in temperature. Thus, wehave discovered a new use for a class of lead compounds in themanufacture of thermistor devices.

In accordance with the present invention, we have discovered athermistor device comprising electrical current leads and a thermallysensitive semiconductor material having the formula:

wherein Z is a member selected from the group consisting of chromium,molybdenum, tungsten, sulfur and mixtures thereof. The thermistor devicecan be made in any conventional shape, but for rapid responses it isusually made as small beads, disks or rods. When used as temperaturesensing devices, the novel thermistors have a negative coefficient ofresistance and can be made from both polycrystalline materials and fromsingle crystals.

The thermally sensitive semiconductor element is broadly made from asingle compound or mixture of compounds which includes the following:

Although the molybdenum, tungsten and sulfur analogs of PbO-PbCrO, showa decrease in resistivity with increasing temperature, their roomtemperature resistance is two to three orders of magnitude higher thanthe chromate. Thus the analogs per se are less desirable as thermistors,but in mixtures they can be used to increase the electrical resistanceof PbO'PbCrO The preferred compound is basic lead chromate (PbO'PbCrOwhich is a congruently melting compound in the PbO-PbCrO, system. Someof its optical properties are well known since the material is anorange-red pigment which darkens on heating. This color change isreversible. The congruent melting behavior of basic lead chromate isadvantageous for single crystal growth from the melt and forsolidification of the material as beads onto metal wires or strips. Thistechnique appears to be a convenient way of making the temperaturesensors. Other methods include hot pressing, swaging composites andforming single crystals.

Since basic lead chromate melts at a temperature of about 928C. in airand is easily contained in platinum crucibles without attack of theplatinum, one can solidify a polycrystalline bead onto wires by rapidwithdrawal of the wires from the melt. Casting into molds is another wayof making the beads. Units of varying bead sizes from about 0.5 to 3 mm.can be made by the wire-dipping technique. Bead size can be increased byrepeated immersion of the bead. Silver, gold and platinum are useful aslead wires for the solidified beads.

Single crystals of basic lead chromate can be pulled from the melt withrelative ease. The details of the chemistry of systems with extensivesolid solution and of the crystal growth process have been reported byRC. DeVries, et. al., Phase Equilibria and Crystal Growth in the SystemsPbO'PbCrO PbO'PbMoO PbO'PbCrO PbO'PbWO and PbOPbCrO PbO'P- b Mat. Res.Bull., Vol. 5, pages 87-100 (1970). The important result is, that notonly can single crystals of basic lead chromate be grown, but extensivesolid solutions in which molybdenum, tungsten and sulfur are substitutedfor chromium can also be made. In preparing speciment for electricalmeasurements, units were made from roughly rectangular pieces of singlecrystals cleaved with two opposite sides parallel, the thickness betweenparallel cleavage surfaces of different crystals ranging from 0.5 mm. to1.0 mm. and each surface to which electrical contact was made was about4-9 mm'.

In making the novel thermistors of the present invention, variousmethods can be used for making 2-point electrical contacts. Theseinclude, for example, (1) simple pressure contact with gold foil againstascleaved surfaces of the crystal; (2) simple pressure contact of goldfoil against a vapor deposited gold or silver coating on cleaved crystalsurface; (3) leads attached to gold or silver coatings by means ofsilver paste bonding material; and (4) soldering of gold or silver leadsto crystals by means of gold-germanium alloy preforms.

The reason why the basic lead chromates are better electronic conductorsthan the analogs listed above is not completely understood, but theproblem is probably best approached by the explanation given by T.W.Lashof, Electrical Conductivity of Lead Chromate, J. Chem. Phys, 1 1,pages l96-202 (1943). This material is an electron-excess conductor, andLashof postulated a supply of electrons within PbCrO, by means of athermally activated dissociation reaction for a small number of chromatemolecules. This dissociation is dependent upon the ability of the Pbions to be reduced to neutral Pb atoms at the same time that the Cr ionis being reduced to Cr. The accompanying oxidation of O" to provides theelectrons for the reduction reactions..Lashof explained that some of thePb atoms thus formed would then be ionized to provide free electrons toaccount for the conductivity.

The suggested mechanism for electronic conductivity in PbCrO, requiresthat some of the 6 ions change to lower valence states. Because of theease with which Cr can do this compared to the other ions, Mo, W, and S,we expect greater conductivity in the basic lead chromate than in theanalogs. This has proved to be the case.

The accompanying FIGURE illustrates the semiconducting properties ofsingle compounds and mixtures of compounds prepared in accordance withthe present invention. I

Referring to the drawing, the resistances in ohms of the semiconductingmaterials made from roughly rectangular crystals cleaved with twoopposite sides parallel and having a thickness of 0.5-1.0 mm. are shownat various temperatures for single compounds: Curve A PbO-PbMoO, andCurve C PbO'PbCrO and for mixtures of these compounds: Curves B, D, Eand F. These curves indicate that the novel thermistors have a negativecoefficient of resistance and that the resistivity of the PbO-PbCrO,compound is substantially lower than the Pb0'PbMoO compound. With regardto the mixtures, the addition of PbO'PbCrO, reduces the resistivity ofthe PbO-PbMoO, (see Curve 13) until a point is reached at which themixture of PbO'PbMoO -PbO'PbCrO' (Curves D, E and F) results inproducing curves which aresubstantially similar to the pure PbOPbCl'O(Curve C).

As'is well known in the art of making thermistors, the semiconductingelement of the present invention can be-encapsulated in conventionalprotective materials, such as polymeric'materials, glasses and ceramics.

Our invention is further illustrated by thefollowing I examples. I

EXAMPLE I A PbO'PbCrO, thermistor was made by the following technique.Powders of 22.32 grams of reagent grade PhD and 32.32- grams of reagentgrade PbCrO, were mixed and then melted together in a platinum crucibleat about l,000C. Two gold wires about 0.5 mm. thick and cm. long andspaced about 1.0 mm. apart, were immersed into the melt to a depth of2-3 mm. Upon withdrawal of the wires from the melt, a droplet which wascooled to a solid bead adhered to the closely spaced wires.

The resistance of the bead and wire unit was then measured as a functionof temperature. This was accomplished by attaching the unit to two wireswhich were threaded into a four-hole ceramic thermocouple protectiontube. The other two holes in the tube held a thermocouple which could befixed in a position within l-2 mm. of the bead unit. This entire unitwas lowered into a tube furnace of the resistance heating type in whichthe temperature could be controlled to :t 2C. The wires supporting thebead unit were attached to a vacuum tube voltmeter or a resistancebridge or a megohm bridge. The vacuum tube voltmeter was standardizedbefore each reading against known resistances. Resistance readings weretaken at a series of temperatures determined from the thermocouple.Final readings were taken after the furnace temperature had equilibratedat the desired temperature. The room temperature resistance of a headunit was 10", but at 550C. the resistance had dropped to about 10 ohmsand to nearly 10 ohms at 850C.

EXAMPLE II A. A single crystal of PbO-PbCrO grown by pulling fromthemelt, was cleaved into thin slices in the thickness range 0.5 to 1.0mm. and small rectangular pieces of about l-2 mm were mechanicallyclamped between two pieces of 0.001 inch gold foil which served aselectrodes. Upon application of heat to the crystal, a marked decreasein resistance from a room temperature value of about 10 ohms was notedin each case and the effect was reversible and reproducible upon coolingand heating, respectively.

When the sample was placed in a furnace with a thermocouple andresistance measurements taken at different temperature, a curve of thetype shown in Curve C of the drawing was obtained. It was found that anaging treating of about 15 hours in the range 500-550C. was effective inincreasing the reproducibility of the results. Because of theirintrinsic homogeneity and greater perfection compared to thepolycrystalline beads, the single crystal thermistor units are morestable than the solidified beads.

B. A single crystal of PbO-PbCrO, was coated with a vapor deposited filmof chromium metal overlaid by vapor deposited gold in order to providean electrical contact. Measurements of resistance versus temperatureafter attaching conducting lead wires to the vapor deposited metallayers showed the same negative temperature coefficient behaviorpreviously described.

C. Gold electrodes were attached to a single crystal of PbO-PbCrO, by asoldering method using a 93 Au-7Ge (wt.%) alloy preform as the solderbetween electrode and crystal. The electrodes, preform and crystal werestacked in the proper sequence vertically "on a support rod and clampedtogether in a furnace.

EXAMPLE III A powder of PbO'PbCrO composition was sintered by hotpressing at 60 kilobars and l,020C. Electrodes were pressed on the endsof the cylindrical sample during the sintering process. The sample was2.5 mm. high and 3.5 mm. in diameter. This unit was connected to avacuum tube voltmeter resistance measuring circuit, and it wasdemonstrated that the material in this sintered form behaved as athermistor with a negative temperature coefficient upon heating EXAMPLEIV PbO-PbMoO, was mixed in varying proportions with PbO-PbCrO andthermistors made from single crystal elements were made as described inExample "A. The compositions of the mixtures in mole percent is setforth in the table below.

Composition PbOPbCrO PbO'PbMOO,

The resistance versus temperature data are shown in the curves of thedrawing.

-' EXAMPLE V The ends of two noble metal wires (about 0.5 mm. thick Xcm. long) held rigidly about 1.0 mm. apart at one end were held in theflame of a gas burner until red hot. The hot wires were then quicklyimmersed into a homogeneous premelted and powdered mixture of 44.64grams of reagent grade PbO and 23.19 grams of reagent grade W0 so thatsome of the powder stuck to the wires. The coated wire ends were thenreheated in the gas flame to about 1,000C. until the powder mixturemelted and formed a droplet of liquid suspended between the two wires.Upon withdrawal from the flame, the liquid solidified to a solid beadjoining the two wires. The process was repeated to obtain the desiredbead size and shape. I

The resistance vs. temperature characteristics of the PbO'PbWO beadthermistors were measured with standard electrical resistance measuringdevices. A Pt-Pt/lO Rh thermocouple inserted into a furnace within l-2mm. of the bead unit was used to measure the temperature which wascontrolled to i 2C. The room temperature resistance of the PbO'PbWO beadthermistor was found to be ohms but the resistance decreasedcontinuously with increasing temperature to 8Xl0" ohms at 570C.

' EXAMPLE VI A droplet of molten PbO'PbSO was solidified onto two goldwires and the room temperature resistance was found to be greater than10 ohms. A negative temperature coefficient of resistance wasdemonstrated by a decrease in resistance to about 1X10 ohms at 570C.

EXAMPLE VlI Resistance (ohms) 570C. 25C. 1 .6X 1 0 7X 1 0 EXAMPLE VlllMixtures of PbO-PbCrO, and PbO'PbSO, in varying proportions were madeinto thermistor units by the bead method. The decrease in resistancewith increasing temperature is demonstrated as shown in the table below.Since the resistance is a function of the Cr /S 0 ratio, this examplealso demonstrates the role of the mixture of compounds in controllingelectrical properties.

Composition (molar ratio) Resistance (ohms) It will be appreciated thatthe invention is not limited to the specific details shown in theexamples and illustrations and that various modifications may be madewithin the ordinary skill in the art without departing from the spiritand scope of the invention.

We claim:

1. A thermistor device having a negative coefficient of resistancecomprising a thermally sensitive semiconductor element of a materialconsisting essentially of a compound having the formula:

wherein Z is a member selected from the group consisting of chromium,molybdenum, tungsten, sulfur and mixtures thereof; and at least twospaced electrical conductors in electrical contact with said element.

2. The thermistor device of claim 1, wherein said material isPbO-PbCrO...

3. The thermistor device of claim 1, wherein said material isPbO'PbMoO.,.

4. The thermistor device of claim 1, wherein said material is a mixtureof PbO-PbCrO, and PbO'PbZ'O wherein Z is a member selected from groupconsisting of molybdenum, tungsten, sulfur and mixtures thereof.

5. The thermistor device of claim 4, wherein Z is molybdenum.

6. The thermistor device of claim 1, wherein said semiconducting elementis in the form of a single crys tal.

7. The thermistor device of claim 1, wherein said semiconducting elementis a polycrystalline material.

8. The thermistor device of claim 1, wherein said semiconducting elementis encapsulated in a protective material.

2. The thermistor device of claim 1, wherein said material isPbO.PbCrO4.
 3. The thermistor device of claim 1, wherein said materialis PbO.PbMoO4.
 4. The thermistor device of claim 1, wherein saidmaterial is a mixture of PbO.PbCrO4 and PbO.PbZ''O4, wherein Z'' is amember selected from group consisting of molybdenum, tungsten, sulfurand mixtures thereof.
 5. The thermistoR device of claim 4, wherein Z''is molybdenum.
 6. The thermistor device of claim 1, wherein saidsemiconducting element is in the form of a single crystal.
 7. Thethermistor device of claim 1, wherein said semiconducting element is apolycrystalline material.
 8. The thermistor device of claim 1, whereinsaid semiconducting element is encapsulated in a protective material.